1. Field of the Invention
The present invention relates to a leakage detecting device for an airtight vessel which is employed for inspecting airtightness of, for instance, a fuel tank of an evaporated fuel gas treating device for an automobile or a piping system.
2. Discussion of Background
Conventionally, a leakage detecting device which is employed in an evaporated fuel gas treating device for an automobile, is constructed as shown in FIG. 2.
FIG. 2 is a construction diagram of a conventional leakage detecting device, which is disclosed in "Technical Support Document Jul. 26, 1991" issued by State of California Air Resources Board.
In FIG. 2, reference numeral 1 designates a fuel tank which is mounted on a car body of an automobile, not shown. Numeral 2 designates a canister which is constructed to adsorb evaporated gas from fuel inside the fuel tank 1, and which communicates with the top air space of the fuel tank 1 through a communicating pipe 3.
Numeral 4 designates an air cleaner for cleaning air sucked by an engine, not shown, and 5, an intake pipe connecting the air cleaner 4 to the engine, in which a throttle valve 6 is provided. Further, a portion downstream of the throttle valve 6 in the intake pipe 5, communicates with the canister 2 through a communicating pipe 7. The communicating pipe 7 is provided with a canister purge valve 8 by which the passage is opened and closed.
Numeral 9 is a pipe for communicating the canister 2 to the atmosphere, which is provided with a canister close valve 10. Numeral 11 designates a cap for sealing up the fuel tank 1 and 12, a canister protection valve connected to the fuel tank 1.
Numeral 13 designates a pressure sensor for detecting a pressure in the air chamber portion of the fuel tank 1 in performing a leakage check, mentioned later. This pressure sensor 13 communicates with the air chamber portion of the fuel tank 1 through a pressure communicating pipe 14, the pressure of which is outputted to a control circuit 15.
The control circuit 15 is constructed to control the opening and closing operation of the canister purge valve 8 and the canister close valve 10, and to perform the leakage check on portions from the fuel tank 1 to the canister purge valve 8 and the canister close valve 10 wherein the evaporated fuel gas flows. The leakage check is performed by decompressing the portions wherein the evaporated fuel gas flows, employing a negative pressure inside the intake pipe 5 and by monitoring a pressure change detected by the pressure sensor 13 during a constant time period.
Next, an explanation will be given of the operation of the above device.
Liquid fuel such as gasoline is stored in the fuel tank 1 and air is enclosed on the top side of the liquid surface of the fuel. It is necessary that the air portion communicates with the atmosphere such that the pressure in the fuel tank 1 does not become a negative pressure to an exceeding degree and air in the atmosphere can be introduced therein when the fuel is consumed.
Conversely, when the fuel tank 1 is heated by an atmospheric temperature elevation, sunlight and the like, the air in the fuel tank expands. Therefore, it is necessary to discharge the air in the fuel tank 1 to the outside. However, the air inside the fuel tank 1 contains the evaporated gas of fuel since the fuel normally contains a volatile component. Therefore, when the fuel containing the evaporated gas of fuel is discharged in the air, air contamination is caused. To avoid the air contamination, the air inside the fuel tank is led to flow to the intake pipe 5 through the communicating pipe 3, the canister 2, the communicating pipe 7 and the canister purge valve 8, from which the air is introduced to an engine and the fuel portion is combusted in the engine.
However, when the evaporated gas is introduced into the engine as above, the air-fuel ratio changes, which exerts a very unfavorable influence on the running of the engine especially when the engine is driven in a low output state. A control is performed to prevent the above drawback such that the air containing the evaporated gas does not flow to the engine when the engine is driven in the low output state. The control circuit 15 determines whether the running state of the engine is in the low output state, and closes the canister purge valve 8 when the engine is in the low output state.
At this moment, the control circuit 15 opens the canister close valve 10. In this way, the air containing the evaporated gas is removed of the evaporated gas by the canister 2 and the cleaned air is discharged to the atmosphere through the pipe 9 and the canister close valve 10.
When the canister 2 stores the evaporated gas, the evaporated gas catching function of the canister 2 is gradually saturated. Therefore, the evaporated gas inside the canister 2 flows into the engine, when the engine is in a high output running state and the canister purge valve 8 is opened, thereby purging the canister 2. When the engine is in the high output running state, air in the atmosphere is drawn into the canister 2 through the pipe 9 by employing the negative pressure inside the intake pipe 5. The evaporated gas in the canister 2 flows to the intake pipe 5 through the communicating pipe 7 and the canister purge valve 8 and is introduced to the engine, along with the air containing the evaporated gas from the fuel tank 1. This is the normal operation in the conventional evaporated fuel gas treating device.
However, a large amount of the evaporated fuel gas can be discharged to the atmosphere when the airtightness is lost wherein the cap 11 is not completely closed or some portion of the evaporated fuel gas treating device is destructed. In the conventional case, a leakage check is performed by employing the pressure sensor 13 to avoid such a disadvantage.
This leakage check is performed by decompressing the total evaporated fuel gas treating system by the intake negative pressure of an engine. The control circuit 15 controls the operations of the canister purge valve 8 and the canister close valve 10, and compares a pressure change quantity outputted from the pressure sensor 13 with a second pressure change quantity when the device is normal.
The control circuit 15 closes the canister close valve 10, when the engine is in the high output running state and the canister purge valve 8 is opened, once in a pertinent period of time, and changes the pressure inside the evaporated fuel gas treating device including the fuel tank 1 and the canister 2, into a negative pressure by employing the negative pressure inside the intake pipe 5. Thereafter, the control circuit 15 closes the purge valve 8 thereby generating a closed system in the evaporated fuel gas treating device, and monitors the output signal of the pressure sensor 13 for a certain duration of time.
At this occasion, the control circuit 15 compares a pressure increasing rate when the device is normal, with a second pressure increasing rate detected by the pressure sensor 13. When a rapid pressure increasing rate is recognized in comparison with the one when the device is normal, the control circuit 15 determines that there is a leakage in the system and alarms the operator. The leakage check has been performed as above.
However, the pressure sensor is generally expensive, and, therefore, there is a limitation in reducing the cost in the leakage detecting device employing the pressure sensor as stated above.